1. Field of the Invention
The present invention relates to a magnetic memory that has a magnetoresistive element for data storage.
2. Description of the Related Art
Magnetic random access memory (MRAM), a type of memory device used in computers, communication devices and other data-processing devices, has attracted great attention. Unlike volatile memories, such as dynamic random access memory (DRAM) and static random access memory (SRAM), in which stored data is lost once the power is turned off, MRAM magnetically stores data, which is retained in the absence of electricity. MRAM offers a faster access speed, higher reliability and less power consumption than other conventional non-volatile memories, such as flash EEPROM and hard disks. For these reasons, MRAM is considered to replace functions of both volatile memories, such as DRAM and SRAM, and non-volatile memories, such as flash EEPROM and hard disks (see, for example, Japanese Patent No. 3466470).
In developing information devices adapted to ubiquitous computing, which, simply put, is a concept that enables data-processing wherever you are, memories are needed that can handle high-speed processing, has less power consumption, and do not lose the stored data when the power is inadvertently turned off. MRAM has the potential to meet all of these requirements and is thus expected to be used in a variety of information devices.
Potential applications of MRAM include cards and portable digital assistants (PDAs) that are carried around in everyday lives. It is not always easy to have available power sources where these devices are used. Thus, in order for the devices to process large quantities of data in such a harsh environment, the already low power consumption of MRAM needs to be further decreased.
A technology for reducing power consumption of MRAM is utilized in magnetic memories described in Japanese Patent Laid-Open Publications Nos. 2000-90658 and 2004-128430. These magnetic memories have a grid of memory cells, each of which consists of a bit line, a word line arranged perpendicular to the bit line, and a tunneling magnetoresistive (TMR) element arranged between the bit line and the word line at the intersection of the two lines. Each memory cell also includes a yoke (magnetic field control layer) that encircles the TMR element and associated bit and word lines. The yoke is formed of a ferromagnetic material having a high magnetic permeability and serves to reduce leakage of magnetic flux from the bit or word line and to focus magnetic flux onto the TMR element. As a result, magnetic fields required for the reversal of magnetization in TMR elements can be generated at low power consumption and magnetic flux can be focused onto the TMR elements.
A TMR element is configured to include a first magnetic layer (sense layer) in which the direction of magnetization is changed by the external magnetic fields, a second magnetic layer that has a fixed magnetization, and a non-magnetic insulating layer sandwiched between the first and the second magnetic layers. A binary data is stored in the element by magnetizing the first magnetic layer parallel or antiparallel to the magnetization of the second magnetic layer.
To decrease the write current in magnetic memories, it is desirable to minimize the coercivity of the first magnetic layer (sense layer) of TMR elements. However, the magnetization of the first magnetic layer with a decreased coercivity can be reversed by mistake by external magnetic fields or magnetic fields generated by adjacent lines, resulting in an increased likelihood of writing errors.
One approach to avoid such writing errors is to cover the entire magnetic memory package with a magnetic shield to block external magnetic fields. Although such a magnetic shield is effective in blocking magnetic fields acting from outside the package, it cannot prevent interfering magnetic fields generated by adjacent lines within the package. It has thus been difficult to eliminate the effect of write magnetic fields generated by individual TMR elements in highly integrated magnetic memories.
The present invention has been devised in view of the above-described problem of the prior art. It is therefore an object of the present invention to provide a magnetic memory in which individual TMR elements are effectively shielded from external magnetic fields.